It's because of self-selection:
There’s a Chronicle of Higher Ed article on researchers who are figuring out why there are so few conservatives in academia. According to Matthew Woessner and April Kelley-Woessner, who teach at Penn State, the issue is simply self selection. Analyzing survey data from college students (public data and their own research), they’ve found that conservative students are less likely to do the things that help you get into grad school (like actively seeking out professors outside of class) and they have a stronger preference for starting a family early. They also found that conservative students are less likely to be interested in the sorts of topics that lead to the PhD degree.
The nice aspect of this research program is that Woessner/Kelley-Woessner tested competing hypotheses, like conservatives get lower grades. The interesting finding is that political moderates get the worst grades. My guess is that people with lower cognitive skills probably can’t clearly distinguish between competing political theories and resort to the middle position. The researchers also test multiple specifications (e.g., self reported ideology vs. policy positions), so the finding appear robust.
The big policy implication is that Horowitz style crusades to forcibly diversify academia are bound to fail. There may be an occasional bit of discrimination against a few conservative academics, but the basic point is that conservatives just don’t want to live the academic life style. Horowitz is throwing a party for people who aren’t showing up!